Some Comments On Bonding By John B. Park II (EAA 10609) 33571 Aqua Dulce Canyon Road Saugus, California

HIS ARTICLE IS limited in scope to the application of T and composite (metal-to-plastic) structures,

advanced metal bonding techniques to metal-to-metal

The word "bonding", to the writer, is synonymous with the word "progress." The word "progress" typically describes a way of life. The Homosapien species has

progressed from caveman to high-speed, high-flying, highstrung modern man. Constantly at war with himself and his neighbor, man displays a competitive spirit which boggles the minds of even the most casual observers. It is the result of this competitive spirit which, in many cases, we call progress. We can see that air racing, in its pioneering, unregulated days, had been a major stimulus to progress in the art of aircraft design. Currently, with overwhelming design restrictions placed on racing planes, progress has been forced to find another source of stimulus. Intelligence has won; a source has been found. A source as unlimited as man's ability to think — the profit motive! Where does bonding fit in? Bonding, as a structural assembly technique, has been around since the beginning of mankind. Cavemen used various forms of glue to hold things together. More recently, most of us as kids have patched up something or other with a fascinating chemical compound in which the primary ingredients are flour and water, and maybe a little sugar to improve the taste. Don't laugh — the stuff really works! For a while. Real progress began when the aerospace industry discovered that increased profits would result if new, improved bonding techniques were applied to vehicle structures. Typical industry claims for bonding include the fact that bonding: * * * * * *

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Improves structural fatigue life Reduces structural weight Adds a vibration, or sound-dampening, quality to a structure Forms an inherent aerodynamic, and cabin or tank seal Reduces corrosion Allows the vehicle designer a greater degree of freedom

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1970

The most noteworthy fact is that when, and by the way only when, the bonding process is used properly all of the industry claims are valid and somewhat conservative. Bonding is really something you can write home to mom about. But don't mention glue. The new word is adhesive! This article is written to introduce major bonding techniques to the home-builder and, at the same time, correct some of the misinformation which has become an outgrowth of the introduction of bonding to small aircraft. What is Bonding?

First, let's define the scope of this paper. We're dealing only with metal-to-metal and composite structures. The

information related will not generally apply to wood structures.

Bonding is the process of joining structural members by surface attachment. Surface attachment is the term applied to molecules adhering (sticking) to each other.

Mating surfaces do not need to be rough, or sanded to allow proper bonding. Just CLEAN! Since surface attachment is a molecule-to-molecule union, we can see that the adhesive cannot be separated from the structural member by any foreign (non-structural) molecules such as dirt, grease,

water, or surface oxidation. Parent metal must mate with the adhesive material, or a bond will not occur. How Does a Bond Work?

Or, how do we use bonding when designing a vehicle? In a word — in shear. Every reader knows what it's like to peel a bandage off skin. Some adhesives are pretty strong in peel loads, but just try to shear a bandage off your skin. Takes nerve, doesn't it? Fig. 1 indicates the nature of a peel load. This is the type of load which a pulley bracket will put on its attachment hardware. Fig. 2 illustrates a typical shear load. This is the type of load found in skin-to-skin lap joints, or skin-to-bulkhead attachments. Adhesives work well in shear.

What is an Adhesive?

Just about every corner hardware store sells a couple of tubes of stuff called "Part A" and "Part B" which, v.hen mixed together in the proper proportions, form a new substance which is totally unusable if it wasn't used before it became unusuable. It hardened. It's called epoxy; and it's expensive, and it's strong, if used properly. Epoxy is available in several forms. The two-part mix-it-yourself system mentioned above is the most common type. Less common, but equally useful is a one-part epoxy adhesive. This is really a two-part, already mixed, which has been frozen to prevent hardening. To use it, simply thaw it to room temperature and use within the time limits set by the manufacturer. In addition to the two liquid types, a rolled-sheet epoxy film is available. This is a single-part epoxy (already mixed) which is rolled into a thin sheet, coiled up for easy storage, and frozen. To use, simply uncoil, cut strips, or chunks, to whatever size or shape needed and sandwich between the structural members. When heated to manufacturers' recommended cure temperatures, the sheet turns to a liquid (wets) and then hardens, forming a strong, lightweight bond.

We can see that adhesives are categorized by type; single-part, two-part, liquid or sheet. In addition, chemical engineers can change the chemical structure of the epoxy to alter its performance in different load environments. Like adding sugar to your kite glue, so that if the kite didn't fly well, at least it tasted good. Thus, we can see that an epoxy resin which is formulated to work well at high temperature may not hold anything together at low temperature. There's even an adhesive made to bond contact lenses to sharks' eyes. Likely this particular formula won't work well in BD-4 wings. Chemical engineers control epoxy-cure environmental requirements. Almost all available epoxy systems will cure (harden) at room temperature, providing the room is in a place of abode other than an igloo. Most epoxy systems will cure to a very high degree of strength if a certain amount of heat, usually 150-250 degrees F is applied. In addition, clamping pressure applied during cure will substantially improve epoxy system performance over structures bonded without clamping pressure. Typical bonding pressures range from 10 psi. to 150 psi. Bonding pressure can be applied by C-clamps with the bonded joint "squished" between pieces of wood. The aerospace industry uses a plastic membrane (vacuum bag) stretched over the bond area. This membrane is sucked down over the structure, and atmospheric pressure (roughly 14 psi.) supplies the bonding pressure. Jim Bede uses band clamps to provide rib-to-spar bonding pressure. The idea of bonding pressure is based on the fact that an adhesive bond works best if it is very thin, not thick with lots of goop running all over the place. Remember that bonding is molecular attachment, not mechanical attachment (like bolts, rivets and glue in wood structures). One molecule in bond thickness is as good as a million molecules. Typical bond thickness for a good bond is .002 .004 in. Thin, eh?

preparation, we're not bonding, but just hardening a

bunch of epoxy.

2. Cure environment — temperature and pressure requirements as set by the adhesive manufacturer. BOND SURFACE PREPARATION

When we made a kite with our secret-formula (flour, water and sometimes sugar) glue, we developed a system, or sequence of operations. When our system was used, a successful kite almost always resulted — except when unexpected variables entered the room, like baby brothers who enjoyed the glue almost as much as we did. Singularly we planned enough glue to satisfy our own appetites during long periods of tedious kite construction. Collectively — that's baby brother, us and the kite — the glue supply generally was exhausted before the kite was finished. We soon learned about planning. No single aircraft-construction technique draws more heavily on planning than does bonding. We shall soon see why. • STRUCTURAL MEMBERS

> ^

LOAD

ADHESIVE

FIGURE 1. I LOAD

ADHESIVE

To recap and intensify what we've just read, consider the following facts: Adhesive bonds work well in shear Adhesive materials are available in a variety of forms and formulae. Chemical engineers, or adhesive suppliers (not the corner hardware store) should be consulted to

PFEL. LOAD

LOAD

FIGURE 2.

SHEAR LOAD

establish which epoxy systems to use.

The two MOST IMPORTANT factors to bear in mind when dealing with adhesive bonding are: 1. Cleanliness — without cleanliness, or proper surface

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We like to think that OUR system which we developed

as kids was picked up and used directly by the aerospace industry. A close examination of industry techniques reveals obvious similarity to our junior engineer approximation of highly classified, and even secret, experiments. Briefly, our system was to:

1. Cut and fit all of the kite parts

2. Mix the glue 3. Glue the kite together 4. Wait for the glue to dry, OR 4a. Eat the kite — depending on wind conditions.

The aerospace industry labels these sequential operations thusly: 1. Pre-fit Clean — this was new to us Prime — if necessary Adhesive lay-up Cure the adhesive

2. 3. 4. 5.

Pre-fit for the Homebuilder

Pre-fit is clecoing the entire structure together before

riveting to make sure everything fits together. Pre-fit is clamping all structural members together before bonding. During pre-fit, the parts may be handled with bare hands. We shall see that when bonding operations

are performed, no structure may be handled by human hands due to possible contamination from skin oil. Pre-fit is doing all final trim and fit operations because none of these operations can be performed after cleaning — the next step. Bonding Surface Perparation — Cleaning

Basically, the cleaning operation is performed in an attempt to produce a bond surface as clean as a surgeon would take into an operating room. Not enough can be said to emphasize the importance of surface cleanliness. For metal parts the steps generally are as follows:

1. Soap and water wash — use a soap like Tide. Use tap water and rinse well. 2. MEK or acetone rinse as a final degrease rinse. Follow immediately with a demineralized water rinse. Do not allow MEK or acetone to dry prior to rinsing with water as these solutions will evaporate and leave a residue film. 3. Where available, the metal should be etched in a solution such as chromic acid or sulphuric acid. Adhesive system suppliers are often able to relate specific chemical requirements for proper surface preparation. Acid etch will remove all surface oxidation leaving bare metal for

bonding. 4. Final demineralized water rinse. Rinse well. Demineralized water is cneap. In-flight structural failures due to poor bonds are expensive. 5. Perform the "Water Break Free" test on all parts cleaned for bonding. Basically the Water Break test is simply passing water (demineralized) over the entire part. Watch for any pools, beads, or rivers which form and indicate dirt or grease. Just think of what water does when it gets on the hood of a freshly waxed car. It forms beads. So will an adhesive. If the part shows any water break, it can be cleaned "locally" at the dirt, or entirely, at the discretion of the builder. It should be remembered that acid etch removes metal and can only be repeated once or twice. For fiberglas parts which are to be bonded to fiberglas or metal, the steps are generally as follows: 32

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1970

Rinse well with MEK, or acetone. Do not allow this

solvent to dry by evaporation. The accepted method is to soak a clean, lint-free cotton rag in the solvent. Wipe this across the surface — one way, with one hand.

Follow immediately (with the other hand) with a dry

CLEAN, lint-free cotton rag. In all surface preparation cases DO NOT TOUCH THE CLEAN PARTS WITH ANY BARE HANDS, TOOLS, or anything which may contaminate them. Handle clean parts using surgical white cotton gloves which are easily available and cheap. Change gloves often, throw away used gloves. In many cases, special holding fixtures will be required. For example, the tube spar in the BD-4 may be held at each

end to eliminate the necessity to touch the structural surface. Bonding Surface Preparation — Priming

Basically, the priming operation is performed to protect the freshly cleaned parts from corrosion. Obviously, this only applies to metallic structural components. It must be remembered that an acid etch bath was recommended for metal parts to remove surface oxidation. The idea, then, is to prevent oxygen molecules from mating to the metal surface. We accomplish this by coating the surface (immediately after cleaning) with a primer which is recommended by the adhesive manufacturer. This primer generally contains two major components: 1. Zinc-chromate for corrosion prevention

2. An organic primer compound which is compatible with

the adhesive to be used.

The organic primer compound works with the adhesive during the adhesive cure process to produce an acceptable bond. The zinc-chromate prevents corrosion (oxidation) before, and after the bond process. Zinc-chromate, if used in the proper paint carrier, will not interfere with the bonding process.

The homebuilder should become familiar with two

major industry standards which apply to the priming process:

1. Time limits between acid etch and priming; typical industry limit is six hours. 2. Primer thickness. An even, smooth coat of primer should be applied to the entire part to a thickness recommended by the manufacturers. Typical thickness may be from .0002 - .0004 in. Primer thickness is critical to bond strength. If the primer is too thin, the zinc chromate will not be effective, and surface oxidation will prevent proper bond adhesion. More important, however, is the effect of primer coat which is too thick. Too much paint will prevent the adhesive molecules from reaching and adhering to the structural surface. The bond strength then becomes dependent upon paint strength, which isn't very strong. The reader, by now, should have recognized the need for planning. We have seen that surface preparation includes the elimination of need to touch the bond surface. The elimination of handling of the bond surface calls for some pretty careful planning, particularly during priming, and subsequent bonding operations. There is no best method to approach the problem, but every possible solution should be considered. The example of holding the BD-4 spars at the ends has been mentioned, and should work well. However, it should be remembered, in this case, that some provision must be made to prevent acid from entering the inside surface of the spar. Otherwise, we will not have much of a spar left. Problems such as this MUST be considered before any attempt is actually made to use the adhesive bonding process in a homebuilt.

Bonded Structure Preparation — Parts Storage

We can all say what we like about the time required to

build an airplane. The most accurate comment which comes

to mind, however, is that one heck of a lot of time is required. This really indicates that many parts are going to

be stored prior to final assembly usage. It can be said that

many, if not all, of our cleaned and primed parts will be stored for some period of time prior to the adhesive application.

Pre-bond processed parts must be stored in a clean,

relatively moisture-free environment. A typical method used to ensure a clean environment is to wrap the parts in kraft or shelving paper. Again, paper is a cheap method and can be used liberally when consideration is made of the potential hazard of an unbonded structure in flight. Wrap all parts and use plenty of tape to keep dirt, moisture and baby brother's hands off of the freshly processed parts. Bonded Structure Preparation — Adhesive Lay-up

The method used to apply the adhesive will depend strictly on the structural application and the type of adhesive used. A doctor's tongue depressor stick makes a fine applicator for the liquid two part and pre-mixed varieties of epoxy.

Kitchen knives are reasonably good application devices, if the proper cleaning process is applied to the tool before

use. Epoxy resins are toxic, however, and all application tools should be discarded after use to protect user health as well as to prevent structural contamination. It is not the intent of the writer to discuss all potential methods of adhesive application. Rather, the critical nature of this process should be stressed. The adhesive must be

applied in a smooth, even, thin coat. No bubbles, or uncoated areas, should be permitted to exist in the bond

area.

The adhesive should not be allowed to exist outside the

bond area. This is to say that a large fillet of adhesive should not be planned into the bond. Excess adhesive material which squeezes out when parts are mated should

be scraped flush with the edge of the part. This is done for two basic reasons:

1. A weight penalty is paid for excessive adhesive material, which could potentially negate any structural progress made by the use of the adhesive. 2. Epoxy resin, when cured, is somewhat porous. The

degree of porosity is a function of bonding pressure and heat applied during cure. Porosity, in itself, is no major

detriment to structural integrity. Moisture migration through the porous substance will, however, lead to

eventual bond degradation due to substratum (bond

surface) corrosion.

We can see that the least amount of epoxy exposed to the external environment is the best approach to a sound

bonded structure. Clean gloves should be worn during the entire bonding operation. The process should be done in a clean, cool, and dry environment. It should be noted that if the quality-control approach to structural bonding is not fully appreciated, another method of vehicle construction must be found. Metal bonding for flight vehicle structures may be used with rivets to produce a very sound vehicle structure. In many structural applications, the number of rivets needed to maintain structural integrity may be significantly reduced. In a few applications no rivets will be needed, as a bond will assume all load inputs, but only if the bond is properly manufactured.

Bonded Structure Preparation — Adhesive Cure

The most straightforward approach to adhesive cure is to follow the adhesive manufacturer's recommendations. Particularly if a high-strength, sophisticated epoxy system is used.

The builder must bear in mind that the strongest bond

will be obtained through the application of heat and pressure to the bond. In the case of the BD-4 wing, band clamps are used on the internal structure. It is within the capabilities of the sophisticated homebuilder to build a vacuum bag that will fit over the entire wing to create a bond pressure for the "panel-rib" structure. A thin sheet of polyethelene or mylar film is draped over the wing structure. The edges of the film are then brought together and sealed with a rolled-up strip of chromate paste. Chromate paste is readily available. Many homebuilders use it to seal the firewall areas of their planes. A tight seal will be required around the support structure that holds the wing during assembly. At any convenient location in the chromate perimeter seal, insert a vacuum cleaner hose. The vacuum will draw the air out of the bag and a static bonding force will be created by atmospheric pressure. The bond force will be a function of the ability of the vacuum source. Often, a good air compressor used in reverse will draw a high vacuum and create the best possible cure environment. The selection of bag material will depend on two major factors:

1. Amount of heat to be applied to the structure.

2. Ability of the bag material to prevent sticking to squeezed-out adhesive. USE NO WAX, GREASE or any other parting agent near any bonded assembly. Entry of any of these materials into the bond will destroy the entire bond. Saran wrap may make a reliable isolation medium to prevent bag sticking. The builder should experiment on test samples to ascertain the proper method for his project. To apply heat, the builder's own genius will often be

displayed. Genius is said to be ten-percent inspiration and

90 percent perspiration. I'll supply the inspiration. Build an oven. You supply the perspiration. Build an oven. Tommy Dempsey of Odessa, Texas built a fine oven by stacking bricks in a corner of his garage. An old gas room heater provided his heat source. This oven is inexpensive and effective. For bonding, a certain degree of heat control is necessary. A few thermometers inserted strategically in the oven should tell the builder what heat settings should be used to maintain the desired environment. The oven method should be applicable to structures as large as BD-4 wings, or even T-18 fuselages. Heat lamps may work well in

place of a gas heater.

Post Bond Comments

Epoxy resin is porous. Exposed bond-line edges should be well sealed. Internal structures may be sealed with a sealant such as Dow Corning "Silastic." This substance can

be applied, in many cases, prior to bond cure since heat improves "Silastic" qualities. If the seal is inside a fuel cell, the seal material should be chosen accordingly. External, exposed bond surfaces can be effectively sealed with paint. In any case, the builder must remember

that environmental exposure of epoxy-bonded structures

should be eliminated. By this time the reader is asking, "Why bother?" Jim Bede's fine little ship is among the first to employ bonding as a structural construction technique. The BD-4 will not be the last ship to use this construction method. If the homebuilt aircraft is to progress to the level of sophistication evidenced by the BD-4, the "Tailwind", and Thorp's little bird, the builder should consider all the variables involved. We just can't add sugar to epoxy. SPORT AVIATION

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